1. Field of the Invention
The present invention relates to a CZ raw material supply method used for forming a molten raw material in single crystal growth by means of a CZ method, and more particularly, to a CZ raw material supply method used in charging a solid raw material in recharge and additional charge.
2. Description of the Background Art
In silicon single crystal growth by means of a CZ method, as well known, poly-silicon in a solid state initially charged in a crucible is heated with a heater to melt. After a raw material melt is formed in the crucible in such a way, a seed is dipped into the molten raw material in the crucible and, starting in this state, the seed is raised while rotating the seed and the crucible, thereby growing a silicon single crystal in the shape of a cylinder at and below the lower end of the seed. As a solid raw material initially charged in the crucible, there are used cut rods, lumps, granules and others, either alone or in combination.
In such a silicon single crystal growth, a solid raw material initially charged in a crucible is molten to decrease its volume; therefore an amount of the molten raw material is limited to be smaller as compared with a volume of the crucible, leading to an unavoidable reduction in productivity. In order to avoid such an inconvenience, it has been contrived in ways to increase a charge amount of a raw material into a crucible, as one of which a technique called additional charge has been developed.
In the additional charge, a solid raw material initially charged in a crucible is molten and thereafter, a solid raw material is additionally charged into the molten raw material in the crucible. As one of modes for raw material charge, there is exemplified a method in which a solid raw material in the shapes of granules/lumps is additionally charged into a molten raw material in a crucible with a raw material supply tube inserted in a furnace (JP A 97 (H9)-208368, JP A 99(H11)-236290 and others). A solid raw material in the shapes of granules/lumps is additionally charged into a molten raw material in a crucible to thereby increase an amount of the molten raw material in the crucible and in turn, use a volume of the crucible effectively, leading to improvement on productivity.
Furthermore, raw material supply called recharge is performed for the purpose to reduce a crucible cost. This is a technique in which after a single crystal is pulled up, a solid raw material is additionally charged into a remaining melt in the crucible to thereby again form a molten raw material at a prescribed amount in the crucible and to then perform pulling-up of a single crystal, followed by repetition of the operations with the result of an increased number of pulling-up times per one crucible. In this technique, additional charge of a solid raw material is recharge, which is implemented in a similar way to the additional charge described above.
In a prior art raw material supply method in which a solid raw material in the shapes of granules/lumps is additionally charged into a molten raw material in a crucible with a raw material supply tube, however, a problem arises that splashing of the molten raw material occur in company with dropping of a solid raw material and flying droplets of the molten raw material are attached onto parts in a furnace to shorten lifetimes thereof, or exert an adverse influence on single crystal growth.
In order to solve a problem of the melt splashing, a method is contrived in which a surface of a molten raw material in a crucible is solidified and thereafter, a solid raw material is dropped thereonto, followed by melting the dropped raw material together with a solidified layer of the surface. In this method, no splashing of a molten raw material occurs whereas an extra power is required to melt the solidified layer, resulting in a problem to increase a power cost.
Incidentally, poly-silicon rods are also used as a solid raw material in additional charge or recharge, but in this case, poly-silicon rods are more expensive as compared with poly-silicon in the shapes of granules/lumps and have a risk of falling off of a part thereof due to cracking.